BOULDER -- The next sunspot cycle will be 30-50% stronger than the last
one and begin as much as a year late, according to a breakthrough
forecast using a computer model of solar dynamics developed by
scientists at the National Center for Atmospheric Research (NCAR).
Predicting the Sun's cycles accurately, years in advance, will help
societies plan for active bouts of solar storms, which can slow
satellite orbits, disrupt communications, and bring down power systems.

The scientists have confidence in the forecast because, in a series of
test runs, the newly developed model simulated the strength of the past
eight solar cycles with more than 98% accuracy. The forecasts are
generated, in part, by tracking the subsurface movements of the sunspot
remnants of the previous two solar cycles. The team is publishing its
forecast in the current issue of Geophysical Research Letters.

"Our model has demonstrated the necessary skill to be used as a
forecasting tool," says NCAR scientist Mausumi Dikpati, the leader of
the forecast team at NCAR's High Altitude Observatory that also includes
Peter Gilman and Giuliana de Toma.

Understanding the cycles

The Sun goes through approximately 11-year cycles, from peak storm
activity to quiet and back again. Solar scientists have tracked them for
some time without being able to predict their relative intensity or timing.

Forecasting the cycle may help society anticipate solar storms, which
can disrupt communications and power systems and affect the orbits of
satellites. The storms are linked to twisted magnetic fields in the Sun
that suddenly snap and release tremendous amounts of energy. They tend
to occur near dark regions of concentrated magnetic fields, known as
sunspots.

The NCAR team's computer model, known as the Predictive Flux-transport
Dynamo Model, draws on research by NCAR scientists indicating that the
evolution of sunspots is caused by a current of plasma, or electrified
gas, that circulates between the Sun's equator and its poles over a
period of 17 to 22 years. This current acts like a conveyor belt of
sunspots.

The sunspot process begins with tightly concentrated magnetic field
lines in the solar convection zone (the outermost layer of the Sun's
interior). The field lines rise to the surface at low latitudes and form
bipolar sunspots, which are regions of concentrated magnetic fields.
When these sunspots decay, they imprint the moving plasma with a type of
magnetic signature. As the plasma nears the poles, it sinks about
200,000 kilometers (124,000 miles) back into the convection zone and
starts returning toward the equator at a speed of about one meter (three
feet) per second or slower. The increasingly concentrated fields become
stretched and twisted by the internal rotation of the Sun as they near
the equator, gradually becoming less stable than the surrounding plasma.
This eventually causes coiled-up magnetic field lines to rise up, tear
through the Sun's surface, and create new sunspots.

The subsurface plasma flow used in the model has been verified with the
relatively new technique of helioseismology, based on observations from
both NSF- and NASA-supported instruments. This technique tracks sound
waves reverberating inside the Sun to reveal details about the interior,
much as a doctor might use an ultrasound to see inside a patient.

Predicting Cycles 24 and 25

The Predictive Flux-transport Dynamo Model is enabling NCAR scientists
to predict that the next solar cycle, known as Cycle 24, will produce
sunspots across an area slightly larger than 2.5% of the visible surface
of the Sun. The scientists expect the cycle to begin in late 2007 or
early 2008, which is about 6 to 12 months later than a cycle would
normally start. Cycle 24 is likely to reach its peak about 2012.

By analyzing recent solar cycles, the scientists also hope to forecast
sunspot activity two solar cycles, or 22 years, into the future. The
NCAR team is planning in the next year to issue a forecast of Cycle 25,
which will peak in the early 2020s.

"This is a significant breakthrough with important applications,
especially for satellite-dependent sectors of society," explains NCAR
scientist Peter Gilman.

The NCAR team received funding from the National Science Foundation and
NASA’s Living with a Star program.

The National Center for Atmospheric Research and UCAR Office of Programs
are operated by University Corporation for Atmospheric Research under
the sponsorship of the National Science Foundation and other agencies.

NCAR scientists have succeeded in simulating the intensity of the
sunspot cycle by developing a new computer model of solar processes.
This figure compares observations of the past 12 cycles (above) with
model results that closely match the sunspot peaks (below). The
intensity level is based on the amount of the Sun's visible hemisphere
with sunspot activity. The NCAR team predicts the next cycle will be
30-50% more intense than the current cycle. (Figure by Mausumi Dikpati,
Peter Gilman, and Giuliana de Toma, NCAR.)